Column grid array or ball grid array pad on via

ABSTRACT

A electronic apparatus and a process for its manufacture are disclosed. The apparatus includes a planar card for accommodating an electronics module package having protruding solder columns and solder joints to mechanically mount and electrically connect the solder columns of the module to the planar card. The planar card includes a first side and a second side, a plurality of wiring lines forming a wiring pattern, and a plurality of vias extending at least partially through the card. Each of the vias includes at least one recessed area extending from one or both sides of the card. The recessed areas extending to a depth within the planar card sufficient to wick the solder joints, and the each of the recessed areas are shaped to provide surface tension to mechanically retain the solder joints.

This application is a Divisional of application Ser. No. 08/782,860,filed Jan. 13, 1997, which is a continuation of Ser. No. 08/364,790,filed Dec. 27, 1994, which applications are incorporated herein byreference.

FIELD OF THE INVENTION

This invention relates generally to surface mount technology in theelectronic packaging area. More particularly, this invention relates toan apparatus and a method for attaching an electronic module package toa planar card, such as a printed circuit board, resulting in increasedcircuit density (connections per area) and increased packagereliability.

BACKGROUND OF THE INVENTION

Conventional methods of mounting electronic module packages ("modules")on printed circuit boards have been achieved with pin grid arrays(PGAs). Typical PGAs are used for mounting devices with large numbers ofpins into printed circuit boards.

The prior art PGAs generally included a module having rigid pinsextending through the plated through holes in the printed circuitboards. These pins are mounted in these plated through holes withconventional solder, which forms a solder fillet having a very highangle.

This PGA exhibits several drawbacks. A major drawback is the limitationon the lead density, as the plated through holes must be of asufficiently large diameter (approximately 0.102 cm) to receive thelarge diameter pins. As a result, these openings occupy space on theprinted circuit board which limits the number of possible electrical(pin-plated through hole) connections, or circuit density, as commonlyreferred to in the art. Additionally, solder of the solder joint, whichattaches and electrically connects the pins to their respective platedthrough holes, forms solder fillets, that extend a substantial distanceupward along the pins at high angles. This structure produces highstress in the rigid pins and substantially negates the ability of thepins to bend, in response to thermal and other stresses. As a result,the pins and solder joints are subjected to high sheer strains,increasing the likelihood of breakage of the connection between the chipand the printed circuit board, potentially shortening the useful life ofthe electronic device employing the printed circuit board.

Additional prior art PGAs, that improve on those discussed above,include printed circuit boards having enlarged openings at the ends ofthe plated non-through holes, on the top side of the printed circuitboards. These enlarged openings receive rigid pins. The pins aresoldered in the openings leaving solder fillets of very high angles. Thebottom side of the printed circuit board includes multiple terminalsdesigned to accommodate wiring to compensate for the space lost by thelarge pin and hole diameters (approximately 1 mm) on the top(pin-receiving) side of the printed circuit board.

This device exhibits the drawbacks discussed above with respect to thehigh stress and breakage problems associated with the solder fillets.Additionally the large pin and hole diameters restrict the wiring to asingle (bottom) side, thus limiting the amount of possible electricalconnections.

Surface mount technology has also been employed with printed circuitboards. Surface mount technology has gained acceptance as the preferredmeans of joining electronic devices together, particularly in high-endcomputers. As compared to more traditional pin connector methods, likethe above discussed PGAs, where a pin mounted to the backside of aceramic module is thrust through a hole in the board, twice the numberof modules can be placed at the same board area. Other advantages suchas smaller component sizes, greater I/O densities, lower electricalresistance, decreased costs, and shorter signal paths have prompted theindustry migration to surface mount technology.

Conventional surface mount technology involves Column Grid Arrays (CGAs)or Ball Grid Arrays (BGAs), which mount to pads, which are in turnconnected to vias. CGAs are integrated circuit chips or modules whichhave a rectangular matrix of contacts for a substrate, such as a printedcircuit board or the like. The contacts are cylindrical columns ofsolder each having one end bonded to the chip or carrier module. BGAsdiffer from CGAs in that they include approximately spherical shapedsolder balls instead of the cylindrical columns. An example of a CGAusing conventional surface mount technology is illustrated in FIGS. 1and 2 labeled "PRIOR ART".

FIG. 1 shows the CGA surface mount pad arrangement on the printedcircuit board 20 with the module 22 (FIG. 2) removed. The top surface ofthe printed circuit board 20 includes a via (plated through hole) 24which receives a solder column 26 (FIG. 2) from the module 22 (FIG. 2).A circuit wire 28, extending from the via 24, electrically connects thevia 24 to a surface mount pad 30. This connection is commonly referredto as a "dog bone" pattern based on its shape.

FIG. 2 shows the mounting of the module 22 to the printed circuit board20 in detail. Solder columns 26 extend from the module 22, and areconnected to their respective surface mount pads 30, by solder joints 32having solder fillets 34 against the solder columns 26. Through thecircuit wires 28 (FIG. 1), electrical contacts are made with the vias24.

This surface mount technology improved on the PGA technology. By usingflexible solder columns with lower angled solder fillets, the printedcircuit boards have longer usable lives. The chances for stress decreaseas the flexible solder columns and solder fillets are able toaccommodate the thermal expansion and contraction associated with thisCGA technology. However, this prior art CGA exhibits a major drawback inthat the "dog bones" and mounting pads occupy significant surface areaof the printed circuit board, limiting circuit density. To bring the"dog bone" arrangements closer together to increase density isproblematic, in that unwanted electrical shorts may occur should thecomponents of these "dog bone" patterns accidentally come into contactwith each other.

SUMMARY OF THE INVENTION

The present invention improves on these prior art technologies throughthe provision of recessed areas having small diameters on a planar cardwhich directly attach to flexible solder columns protruding from anelectronics module package ("module"). The small diameter recessed areasand direct connections use less surface area of the planar card and assuch provide increased circuit density and allow for wiring on bothsides of the planar card.

The invention comprises a planar card for accommodating an electronicsmodule package having protruding solder columns and solder joints tomechanically mount and electrically connect the solder columns of themodule to the planar card. The planar card includes a upper side and alower side, a plurality of wiring lines forming a wiring pattern, and aplurality of vias extending at least partially through the card. Each ofthe vias includes at least one recessed area extending from one or bothsides of the card. The recessed areas extend to a depth within theplanar card sufficient to wick the solder joints, and the each of therecessed areas are shaped to provide surface tension to mechanicallyretain the solder joints. A process for manufacturing the invention isalso disclosed.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention will be described with reference to theaccompanying drawings, wherein like reference numerals identifycorresponding or like components.

In the drawings:

FIG. 1 is a top view of a printed circuit board of the prior art withthe module removed;

FIG. 2 is a cross sectional side view of the of the prior art;

FIG. 3 is a cross sectional side view of the present invention;

FIG. 4 is a top view of a planar card of the present invention with themodule removed; and

FIG. 5 is a cross sectional side view of a second embodiment of thepresent invention.

DETAILED DESCRIPTION OF THE DRAWINGS

FIGS. 1 and 2 detail the prior art and have been discussed above.

FIGS. 3 and 4 show the electronic apparatus 40 of the present invention.The apparatus 40 includes a planar card 42, preferably a multilayerprinted circuit board or the like (a single layer printed circuit boardis also permissible), with a via 44 (plated through hole), preferablyextending through the planar card 42, from the upper side 46, to thelower side 47. The via 44 includes a body 48 and terminates in arecessed area 50 (depression) at the upper side 46. A solder column 52,extending from an electronics module package ("module") 54, is receivedin the recessed area 50. A solder joint 56 mechanically mounts andelectrically connects to a solder column 52 in the recessed area 50.While the module 54 illustrated is a CGA, it could also be a BGAprovided that a solder ball is used instead of the solder column 52.While only a single solder column 52 is shown in a single via 44 on theplanar card 42, this is for purposes of illustration only, as theapparatus of the invention 40 involves planar cards 42 having multiplevias 44 for accommodating a corresponding number of solder columns 52,or solder balls if a BGA is

The via 44 includes a plated surface 57 along the inner walls, thatpreferably extends the entire length of the via 44, to the recessed area50 and onto the upper side 46 of the planar card 42, and to the lowerside 47 of the planar card 42, forming lands 58, 59 on the respectivesides. Although this arrangement of lands 58, 59 on both sides of theplanar card 42 is preferred, lands on one or both sides of the card arenot required.

Alternately, the via 44 may be a "blind via." Blind vias do not extendentirely through the planar card 42. They may include the recessed area50 and a portion of the via body 48, or the recessed areas 50 alone.

The plated surface 57 is of an electrically conducting metal such ascopper or other suitable electrically conducting material. The platingis performed by conventional techniques, well known to those skilled inthe planar card and printed circuit board art.

The recessed areas 50 of the via 44 include depressions extending fromthe surface formed by a side, preferably the upper side 46 of the planarcard 42. Specifically, the recessed area 50 includes outwardly extendingshoulders 60, that are frustoconical shaped, that have a dihedral angleof approximately 120 degrees.

This frustoconical shape, and shallow depth (approximately 0.1 mm to 0.2mm) into the planar card 42 of the recessed area 50, utilize surfacetension on the molten solder composition (during reflow, as detailedbelow) to compete with the via 44 to retain the solder composition in acompact solder joint 56 as the solder composition cools. In other words,the recessed area 50 wicks the solder joint 56, keeping the soldercomposition on the plated surfaces 57 of the recessed areas 50,preventing the solder composition from spreading into the land to linespaces 62 between the vias 44 on the planar card 42. By inhibiting thespread of the solder composition of the solder joint 56, unwantedelectrical contacts, which could damage the module 54 through shorts,are avoided.

Additionally, the shape and depth of the recessed area 50 allow thesolder joint 56 to form solder fillets 64 having a low angle(approximately 10 degrees from the horizontal). These low angle solderfillets 64 maximize ductility between the solder joint 56 and the soldercolumns 52, to avoid stress fatigue embrittlement.

The solder joint 56 includes a solder composition of a standard flux ina eutectic mix (approximately 63% tin and 37% lead). Other soldercompositions include a mixture of tin/bismuth or other solder alloysknown to those skilled in the art.

The solder columns 52 on the module 54 (only one solder column 52 isshown in FIG. 3) are preferably compliant, in that they are capable offlexing to absorb stress built up between the via 44 and the module 54.This compliance is achieved in part by the solder columns 52 having aheight approximately five times its width (or diameter). This heightallows for an increased bending moment of the solder column 52, suchthat it can expand and contract at rates different than those for themodule 54 to compensate for various stresses and strains at the solderjoint 56. The solder columns 52 may be made of materials such as amixture of approximately 90% tin and 10% lead, copper, KOVAR® (CarpenterTechnology Corporation, Reading, Pennsylvania), or the like. The solderjoints 56 and solder columns 52 of a single apparatus 40 may be of thesame or different materials.

FIG. 4 shows the planar card 42. The recessed areas 50 provide a directattachment of the solder columns 52 (FIG. 3) on the module 54 (FIG. 3),thus eliminating the need for surface mount pads 26 (FIGS. 1 and 2).Additionally, further space saving is achieved on the planar card 42 asthe diameters of the recessed areas 50 are small (approximately 0.7 mm),and the diameters of the via bodies 48 are correspondingly small(approximately 0.3 mm). This allows for a high circuit densityarrangement of connections, and permits wiring on both the upper side 46and the lower side 47, and multiple internal signals and voltage planeson the planar card 42. The increase in circuit density achieved istypically four or more times greater than that of the prior art PGA's orsurface mount technology.

Turning now to FIG. 5, there is shown a second electronic apparatus 70of the present invention. The electronic apparatus 70 is the same as theelectronic apparatus 40 described in FIGS. 3 and 4 above, except asindicated below.

The apparatus 70 includes a planar card 72, similar to the planar card42 discussed above in FIGS. 3 and 4. The planar card 72 has upper andlower sides 74, 75, and a via 76 extending between the sides 74, 75. Thevia 76 includes a body 78 and terminates in a cylindrically shapedrecessed area 80, formed by a depression in the upper side 74 of theplanar card 72. The via 76 may also be a "blind via" (discussed above).

A plated surface 81, similar to that described above, preferably extendsthe entire length of the via 76, to the recessed area 80 and onto theupper side 74 of the planar card 72, and to the lower side 75 of theplanar card 72, forming lands 82, 83 on the respective sides. Althoughthis arrangement of lands 82, 83 on both sides of the planar card 72 ispreferred, lands on one or both sides of the planar card 72 are notrequired.

The recessed area 80 is formed by outwardly extending shoulders 84, thatbend upward at approximately right angles toward the upper side 74 ofthe planar card 72. The recessed area 80 accommodates a solder column52a that extends from a module 54a. A solder joint 86 mechanicallymounts and electrically connects the solder column 52a in the recessedarea 80 of the via 76. While only a single solder column 52a is shown ina single via 76 on the planar card 72, this is for purposes ofillustration only, as the apparatus 70 of the invention involves planarcards 72 having multiple vias 76 for accommodating a correspondingnumber of solder columns 52a, or solder balls if a BGA is used.

The cylindrical shape and shallow depth (approximately 0.1 mm to 0.2 mm)of the recessed area 80 into the planar card 72, utilize surface tensionon the molten solder composition (during reflow, as detailed below) tocompete with the via 76 to retain the solder composition in a compactsolder joint 86 as the solder composition cools. In other words, therecessed area 80 wicks the solder joint 86, keeping the soldercomposition on the plated surface 81 of recessed area 80, preventing thesolder composition from spreading into the land to line spaces 88between the vias 76 on the planar card 72. By inhibiting the spread ofthe solder composition of the solder joint 86, unwanted electricalcontacts, which could damage the module 54a through shorts, are avoided.

Additionally, the shape and depth of the recessed area 80 allows thesolder joint 86 to form solder fillets 90 having a low angle (less thanten degrees from the horizontal). These low angle solder fillets 90maximize beam moment of solder columns 52a, which are compliant andserve to reduce stress fatigue on the solder joint 86 and the soldercolumn 52a.

The electronic apparatus 40, 70 of the present invention are made byfollowing preferred method. Initially, a planar card having vias orplated through holes, terminating in shallow recessed areas(depressions), is made by conventional printed circuit board fabricationtechniques. A stencil, having openings with diameters corresponding theshallow recessed areas is placed over the planar card, such that thestencil openings are aligned with the recessed areas on the planar card.The stencil serves as a mask over the planar card.

Solder paste is then screened onto the planar card with a metal orrubber squeegee blade. The solder paste enters the aligned openings andrecessed areas, subsequently filling them. Excess solder paste isremoved at the same time from the stencil by moving the squeegee bladeor the like over the upper surface of the stencil. Each of the recessedareas facilitates a larger volume of solder paste, to accommodate thesolder requirements for both the solder joint and the solder column,which may be thieved by the via. Additionally, the use of the stencilallows for the solder paste to reach a certain thickness and volumewhich is approximately double or triple the normal solder volume intothe recessed areas than would have been achieved without the recessedareas.

Once the screening step is complete, the stencil is removed. A modulewith solder columns, corresponding to the recessed areas, now filledwith solder paste in the planar card, is joined to the planar card. Thesolder paste is then reflowed by processes well known to those skilledin the art. During the reflowing step, the recessed areas providesufficient surface tension to maintain the molten solder between thesolder columns of the module and the recessed areas. The reflowing steppreferably occurs in a convection reflow oven, IR oven, vapor phasereflow machine or the like, in various atmospheres such as nitrogen, airor the like. Once the reflowing step is complete, the card is cooled.

While the invention has been described with reference to preferredembodiment, it will be understood by those skilled in the art thatchanges may be made without departing from the spirit and scope of theinvention.

What is claimed is:
 1. A method of mounting an electronics module on aplanar card, comprising:a) providing an electronics module having anarray of solder protrusions from one surface thereof for formingelectrical connections; b) providing a planar card having vias and awiring pattern, said planar card further having an array of shallowrecessed areas for placement of said electronics module, wherein saidshallow recessed areas each have a depth between 0 and 0.2 millimeters;c) screening a solder paste onto the planar card, each of the recessedareas facilitating a high solder volume to accommodate the solder pastein the recessed areas as well as the solder paste that may flow intoeach of the vias; d) holding the array of solder protrusions in contactwith a corresponding array of shallow recessed areas formed in the cardand electrically connected to the wiring pattern; e) reflowing thesolder paste to form an array of solder joints between the solderprotrusions and the wiring pattern at the recessed areas, the solderjoints being held in place by surface tension between the solderprotrusions and the shallow recessed areas; and f) cooling the planarcard to solidify the solder joints in place at the recessed areas withthe module spaced above the planar card for movement in a directionparallel to the planar card during thermal cycling of the module.
 2. Themethod of claim 1, wherein the solder paste is screened onto the planarcard by;a) placing a stencil including openings onto the planar card,the stencil openings in line with the recessed areas; b) applying solderpaste into the aligned stencil openings and recessed areas; and c)removing any excess solder paste from the stencil.
 3. A method ofmounting an electronics module on a planar card, comprising the stepsof:a) providing an electronics module including a plurality of solderprotrusions, each protrusion for forming a respective electrical contactbetween an electrical conductor within said electronics module and acorresponding electrical conductor on a planar card; b) providing aplanar card having a first surface and a plurality of depressionsextending from the first surface into the planar card, wherein thedepressions each have a depth between 0 and about 0.2 millimeters; c)aligning the solder protrusions with the depressions of the planar card;and d) forming a solder joint in each depression for securing the solderprotrusion to the planar card and forming a plurality of electricalconnections, each connection being through a respective one of saidsolder protrusions.
 4. The method according to claim 3, furtherincluding placing a solder paste on the first surface of the planar cardprior to aligning the column with the depression.
 5. The methodaccording to claim 4, wherein forming the solder joint includes flowingthe solder paste.
 6. The method according to claim 5, wherein formingthe solder joint includes solidifying the solder joint after flowing thesolder paste.
 7. The method according to claim 3, wherein the depressionis frustroconical.
 8. The method according to claim 3, wherein thedepression includes a side surface extending from the first surface ofthe planar card at an obtuse angle relative to the first surface of theplanar card.
 9. The method according to claim 3, wherein the depressionis cylindrical.
 10. The method according to claim 9, wherein thecylindrical depression includes a side surface extending at a rightangle from the first surface of the planar card and a base surfaceextending at a right angle from the side surface of the depression. 11.The method according to claim 3, wherein providing the planar cardincludes providing a via extending from the depression toward a secondsurface of the card and aligning the column with the depression includesextending part of the column into the depression without extending thecolumn into the via.
 12. The method according to claim 3, whereinforming the solder joint includes forming a portion of the solder jointin a via beneath the column.
 13. The method according to claim 3,wherein forming the solder joint includes forming at least a portion ofthe solder joint on the first surface with an upper surface anglebetween 0 degrees and 10 degrees.
 14. The method according to claim 3,wherein the solder protrusions are cylindrical solder columns.
 15. Themethod according to claim 3, wherein the solder protrusions are solderballs.